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CM: Large-Scale Geothermal Heat Pump (LSGEOHP)

Introduction

👉 Click here to access the online tutorial for this module

The integration of large-scale heat pumps in geothermal projects has gained increasing interest over the last years. Here, we provide a brief overview on the working principle of heat pumps and their role in geothermal energy systems. The page explains how heat pumps can be integrated with deep and medium-deep geothermal sources. In addition, it serves as a guide for using the corresponding calculation module (CM-LSGEOHP).

The CM-LSGEOHP calculation module is designed as an early-stage evaluation tool to estimate the expected thermal output, coefficient of performance (COP), and investment costs of large-scale geothermal heat pumps. The results provide a first rough guidance and must be critically assessed, as they are based on simplified assumptions and are not intended to replace detailed project-specific analyses.

Working principle of heat pumps

Heat pumps transfer heat from a low-temperature source to a higher-temperature sink by using external energy, normally electricity. The basic components of a compression heat pump* include an evaporator, compressor, condenser, and expansion valve, as shown in the process flow diagram in Figure - Working Principle Heat Pump. In the evaporator, heat is absorbed from the geothermal brine, causing the refrigerant to evaporate. The vapor is then compressed, which increases both temperature and pressure. In the condenser, the heat is transferred to the heat sink, for example, a district heating network (DHN), before the refrigerant is expanded and the cycle repeats. The simplified T-s diagram illustrates the thermodynamic processes of evaporation, compression, condensation, and expansion. The Sankey diagram in Figure - Working Principle Heat Pump further highlights the energy flows, demonstrating that only a relatively small amount of electrical energy is needed to transfer a much larger amount of thermal energy.

Figure - Working Principle Heat Pump

Working_principle_heat_pump

The performance of a compression heat pump is characterised by the coefficient of performance (COP), which is the ratio between the useful heat output, thus, the heat supplied to the DHN, and the required input to drive the heat pump, which is the electricity demand of the compressor. Therefore, a COP of e.g. 4 means that by using 1 kWh of electricity, 4 kWh of useful heat at a sufficiently high temperature level can be supplied. Consequently, the higher the COP is, the lower your operational costs (caused by the electricity consumption). However, due to the laws of thermodynamics, the theoretically achievable COP depends on the temperature lift of the heat pump; thus, the difference between the cold temperature source, which is used as an energy source for driving the heat pump and the temperature sink (e.g. the DHN). The higher the temperature lift, the lower the theoretical achievable COP, as shown in Figure - Heat pump Coefficient of Performance (COP). In theory, the so-called Carnot COP represents the theoretical upper limit of the COP; however, in reality, actual heat pumps can achieve a COP of e.g 0.4 to 0.5 of the Carnot COP.

Figure - Heat pump Coefficient of Performance (COP)

COP_Heat_pump

Next to compression heat pumps, also absorption heat pumps and absorption heat transformers are a promising technological concept. Those absorption systems use heat to drive the system, resulting in a significantly lower power demand. Nevertheless, due to the industry's strong focus on compression heat pumps, this section only addresses those systems.

Large-scale heat pumps in the context of geothermal projects

As shown in Figure - Geothermal Scenarios , large-scale heat pumps can be applied for two purposes:

  • Either increasing the thermal capacity of a geothermal heating project
  • or enabling the utilisation of insufficient low geothermal production temperatures.

Direct use without heat pump: In the first scenario, the geothermal brine is produced at a sufficiently high temperature (around 110 °C) to directly supply the district heating network (DHN) at the required temperature (e.g., 105 °C). After transferring heat to the DHN, the cooled brine is reinjected into the reservoir. This setup is highly efficient and straightforward, as it does not require any additional boosting technologies. However, it is only feasible when the geothermal source consistently delivers high production temperatures.

Capacity increase with the heat pump: In the second scenario, the geothermal production temperature remains high, but a high-temperature heat pump (HTHP) is added to increase the total heat extraction from the geothermal source. After direct heat transfer to the DHN, the cooled brine, which still contains usable thermal energy, is further cooled by the heat pump. The heat pump upgrades this remaining lower-temperature heat, thus increasing the overall capacity of the system without needing to drill additional wells or increase flow rates.

Utilization of low production temperatures with the heat pump: In the third scenario, the geothermal production temperature is too low (e.g., 85–90 °C) to directly supply the DHN at the required temperature. Therefore, a high-temperature heat pump is essential to upgrade the geothermal heat to DHN supply conditions. The heat pump extracts energy from the moderately warm geothermal brine and boosts it to meet the DHN supply temperature requirements. Without the heat pump, the geothermal resource would not be usable for the DHN in this case.

Figure - Geothermal Scenarios

Geothermal scenarios

Input parameters

Direct-use calculation module:

Input Name Unit Description
Geothermal brine mass flow rate kg/s Mass flow rate of the geothermal brine.
Geothermal production temperature °C Temperature of the geothermal fluid at the production wellhead.
Geothermal injection temperature °C Temperature of the geothermal fluid when reinjected into the reservoir. In case of a direct-use without a heat pump, the minimal achievable injection temperature is limited by the DHN return temperature.
DHN supply temperature °C Required supply temperature for the district heating network.
DHN return temperature °C Return temperature of the district heating network. Mainly determined by the DHN's customer structure.

Capacity increase calculation module:

Input Name Unit Description
Geothermal brine mass flow rate kg/s Mass flow rate of the geothermal brine.
Geothermal production temperature °C Temperature of the geothermal fluid at the production wellhead.
Geothermal injection temperature without a heat pump (optional) °C Temperature of the geothermal fluid when reinjected into the reservoir without installing a heat pump. Used to estimate additional heat extraction potential.
Minimal feasible geothermal injection temperature °C Lowest achievable injection temperature considering operational or reservoir constraints.
DHN supply temperature °C Required supply temperature for the district heating network.
DHN return temperature °C Return temperature of the district heating network. Mainly determined by the DHN's customer structure.

Temperature Increase calculation module:

Input Name Unit Description
Geothermal brine mass flow rate kg/s Mass flow rate of the geothermal brine.
Geothermal production temperature °C Temperature of the geothermal fluid at the production wellhead.
Geothermal injection temperature °C Temperature of the geothermal fluid when reinjected into the reservoir after heat extraction.
DHN supply temperature °C Required supply temperature for the district heating network.
DHN return temperature °C Return temperature of the district heating network. Mainly determined by the DHN's customer structure.

Sample Run

The LSGEOHP tool can be used in two ways: Either by using it directly on the SAPHEA page (similar to Geophires, etc.) or as a stand-alone HTML file, which can be directly executed in your browser.

Access the tool in the SAPHEA decision support toolbox: Click here

Access the tool in your browser: Click here

In Figure - Landing Page you see how to access the CM within the SAPHEA decision support tool by selecting "Calculation modules" and then "CM-LSGEOHP" and the landing page of the stand-alone calculation tool.

Figure - Landing page of the stand-alone calculation tool

LSGEOHP_Tool_landing_page

For better clarity, the following steps are explained using the stand-alone calculation tool. Once the desired calculation module is selected, the necessary input fields appear (Figure - Input).

Figure CM Large-scale HP - Input

Example supply input

Once all necessary data are filled in with plausible values (otherwise red warnings will appear), the calculation button gets enabled and the user can start the calculation. The results will be displayed in a short section at the end of the page, summarizing the key results, such as heat output from the large-scale heat pump, its COP or the necessary investment costs (see Figure - Results).

Figure CM Large-scale HP - Results

Results_section_example

How To Cite

Ludwig Irrgang, Amedeo Ceruti, Christopher Schifflechner, Ali Kök in SAPHEA-Wiki, CM Large-scale geothermal heat pump.

Authors And Reviewers

This page is written by Ludwig Irrgang, Amedeo Ceruti and Christopher Schifflechner Technical University of Munich.

This page is reviewed by Ali Kök EEG-TU Wien.

In case of any questions, please reach out to Ludwig Irrgang: ludwig.irrgang(at)tum.de

License

Creative Commons Attribution 4.0 International License

This work is licensed under a Creative Commons CC BY 4.0 International License.

SPDX-License-Identifier: CC-BY-4.0

License-Text: https://spdx.org/licenses/CC-BY-4.0.html

References and further literature

Acknowledgement

We want to convey our deepest appreciation to the HORIZON Europe Actions SAPHEA Project (Grant Agreement number 101075510), which co-funded the present investigation.

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